This invention relates to a path measuring instrument of the type comprising a scale which defines both a measuring division and a reference track, wherein the reference track defines a plurality of reference marks, and wherein means are provided for scanning the measuring division and the reference track in order to generate output signals which are evaluated in an evaluating means. As used herein, the term "path measuring instrument" is intended to encompass both length and angle measuring instruments.
Path measuring instruments of the general type described above are well known to the art. For example, as disclosed in West German DE-OS No. 24 16 212, it is a known practice to provide a separate reference track alongside an incremental measurement division in an incremental length or angle measuring device, wherein the reference track defines a plurality of reference marks which serve as absolute position markers. In the system disclosed in the above-identified West German patent application, the spacing between individual pairs of reference marks varies from one pair to another. When the incremental division of the scale is scanned, the actual spacing between any two reference marks can be determined from the sum of the increments counted between the two reference marks. This value is a measure of the separation of the two reference marks, and therefore serves to identify the reference marks in question. Thus, in order to determine the identity of any given reference mark and its absolute position, two adjacent reference marks must be scanned. This process can be complicated and time-consuming if, for example, the two adjacent reference marks are separated by any considerable distance. Moreover, in the event the measuring instrument erroneously counts the increments between two reference marks, a false measurement of the separation between the reference marks is determined, and thereby false absolute values may be formed.